The carbostyril derivatives represented by the general formula (1) are known to be useful as an anti-ulcer agent (JP-B-63-35623). The representative example of the derivative, 2-(4-chlorobenzoylamino)-3-(2-quinolon-4-yl)-propionic acid, is a commercially available medicine, but the formulation thereof is limited to a solid formulation for oral administration or a suspension type liquid formulation (an ophthalmic suspension, an enema, and a gargle) since the compound is very little soluble in water. Suspension type liquid formulations have several problems in the preparation, such as difficulty in keeping of uniformity of contents; necessity of controlling the particle distribution; necessity of use of a suspending agent, a dispersing agent and the like; impossibility of the terminal sterilization by steam or the sterilization by filtration; etc. On the contrary, formulations in the form of a solution have some advantages such as rapid absorption compared with solid formulations and suspension type liquid formulations, and hence it is desirable to formulate the carbostyril derivatives in the form of a solution, such as injections, ophthalmic solutions, oral solutions, enemas, gargles, ear droppers, nasal drops, and external preparations.
The carbostyril derivatives represented by the above general formula (1) and the preparation thereof are disclosed in JP-B-63-35623. It is also known that the carbostyril derivatives are formed into a bismuth salt thereof, a carboxylate-bismuth complex thereof, and a salt with a diamine derivative or a piperazine derivative (WO 95/12579 and JP-A-8-295673). However, such salts also have low solubility in water.